Electrolytic process for production of titanium



Aug. 19, 1958 c. J. CARIGNAN 2,848,395

ELECTROLYTIC PROCESS FOR PRODUCTION OF TITANIUM Filed April 29, 1952 INVENTOR CHARLES J. CARIGNA N Y j k ATTORNEY- ELECTRGLYTTC PROCES FOR PRODUCTION F TITANIUM Charles J. Carignan, Thornbury Township, Chester County, Pa, assignor to E. I. du Pont deNernours and Company, Wilmington, Del., a corporation 'of Dela- Ware Application April 29, 1952, Serial No. 285,093

6 Claims. (Cl. 204-60) This invention relates to metal production and more particularly to novel methods for obtaining various metals by electrowinning techniques. More specifically, it relates to a novel electrolytic method for preparing a metal such as titanium.

Many of the methods used in electrowinning common metals are not applicable for various reasons to titanium metal production. Aqueous electrolytic schemes for obtaining titanium fail to yield a metal product of acceptable quality due to the great reactivity of titanium. Organic media electrolytic methods for producing the metal are also ineffectual for use because of the inability to obtain conducting solutions containing titanium ions which will deposit the metal when an electrical current is applied. Molten salt electrolytic schemes of early date failed to recognize the stringent quality requirement necessary for ductile metal. Various attempts have been made to dissolve oxidic compoundsof titanium in molten salts containing fluorides, the resulting compositions being then electrolyzed to obtain titanium metal. The presence of oxygen in the system is objectionable because of the great avidity of titanium for oxygen. Because titanium ions may exist in several valency states, the requirement also exists that cathode reduction products must not migrate into the anode zone and be reoxidized. This requirement has led to the use of diaphragms, usually of oxide refractories, which have proved bulky and tend to add objectionable impurities to the system. The electrolytic path was also lengthened and electrical power was lost to result in a very inefficient and expensive operation. From this brief summarization of the deficiencies of prior electrolytic methods for recovering titanium metal through reduction of its various compounds, the urgent need for new concepts and methods to successfully and economically electrowin the metal is apparent.

It is accordingly among the objects of this invention to overcome the foregoing and other disadvantages which characterize prior methods for electrolytically producing metals and to provide novel and efiective methods for attaining such objects. A particular object of this invention is to produce titanium metal by the electrolysis of a molten salt system. A further specific object is to provide a novel, eflicient and economic electrolytic process for manufacturing titanium metal wherein resort to a diaphragm cell is not required. Other objects will be apparent from the ensuing description of my invention and the accompanying diagrammatic drawing illustrating one embodiment of the invention.

These and other objects are realized in this invention which broadly comprises electrolyzing, with an inert anode, a fused salt anolyte which is substantially immiscible in and ditferent in density from the catholyte, said anolyte comprising at least one halide of a metal more electropositive than the titanium to be deposited, said catholyte comprising a fused salt solution of a halide of titanium in an alkali metal halide, and separately recovering the anode and cathode products resulting from the electrolysis.

2,848,3h5 Patented Aug. 19, 1958 ice In a more specific and preferred embodiment, the invention comprises preparing titanium metal by electrolyzing with an inert anode and a titanium cathode, a fused salt anolyte of magnesium and sodium chlorides which are substantially immiscible in and lighter in density than the catholyte, said catholyte comprising a fused salt solution of a titanium chloride having a chlorine-to-titanium ratio of between about 2:1 and 3:1 in sodium chloride, and thereafter separately recovering the resulting anode and cathode products.

In accordance with my discovery, a method is provided which successfully separates the anode and cathode zone without requiring a diaphragm type cell. This separation is accomplished by utilizing the interface between two immiscible fluids which serve as the anolyte and catholyte compositions. I have discovered that within the temperature range from 450 C. to about 700 C. it is possible to establish a system containing, as a dense phase, a solution of a lower'subchloride of titanium, such as TiCl Ticl or mixturesof titanium subchlorides, dissolved in sodium chloride beneath a lighter layer of magnesium chloride-sodium chloride compositions. The titanium chloride composition can vary in chlorine-totitanium ratio between the limits of about 2:1 to 3:1, and useful solutions of these materials can contain from about 48 to mol percent of sodium chloride. The titanium subchloride may be prepared in situ or in another step in accordance with known procedures. For example, it can be obtained by reducing titanium tetrachloride with a moderate reducing agent such Zn, Al, Si, Ti, Fe, H etc., or by resorting to a more active form of reducing agent, such as alkali (sodium, potassium, lithium) or alkaline earth metals (calcium, barium, strontium) under controlled conditions, e. g., by flow type reactions Where proportions of the reactants are controlled. The titanium subchloride may be collected, separated from undesired byproducts, or further purified if required. The titanium-containing solution is prepared using due precautions to prevent contamination by deleterious impurities. This titanium-containing solution is to be used as the catholyte in my invention. Anolyte compositions of sodium chloride-magnesium chloride content may vary from about 40 to 70 mol percent of sodium chloride, but a preferred and extremely useful solution is the eutectic composition of about 60 mols percent sodium chloride which melts at about 450 C.

Since the operating temperatures useful in my inven tion are quite low, experiments illustrating my invention may be conducted in glass vessels. The following examples are illustrative of certain modes of applying the invention but are not to be construed as in limitation of its underlying principles and scope.

Example I 2, 45 cm. long, and sealed at one end 3. The tube was clamped vertically in an electrically heated furnace. A section of 10 mil. tungsten wire was sealed through the closed bottom 3 of the tube to serve as a cathode d, and the open top 5 was closed by a neoprene stopper 6 through which was inserted a A" carbon rod as an anode 7. Provisions 8 were made for flushing out the anode gases and for collecting them in a trap. The cell was flushed with an inert gas 8, and a quantity of a titanium lower chloride-sodium chloride mixture (60 mol percent NaCl and-Cl/Ti ratio 25:1 in the titanium salt) was added sufficient to give a molten catholyte 9 about 8 cm. deep. This was fused. and allowed to resolidify.

Then a quantity of freshly prepared sodium chloride- =12! anolyte and the cell and its contents were heated to about 600 C. The carbon anode 7 was inserted into the upper salt layer 10 and electrolysis was begun.

The source of current was a selenium rectifier. cell was found to draw 3 amp. at 8 volts. The upper salt layer became yellow at first but soon became waterwhite and clear. The only anode product was chlorine. Even though bubbles of chlorine evolved at the anode kept the anolyte well stirred, there was no evidence that lower titanium chlorides were diifusing into the upper salt layer. The electrolysis was carried out for about 2 hours. There was a diminution in volume of the electrolyte during this time. The cathode current density was calculated to be about 350 amps. per sq. dm.

When cool, the cell was broken open and the electrolyte examined. The upper salt layer was pure white and free of titanium lower chlorides. The metallic titanium product from the electrolysis was found to be partly in the form of fines and partly in the form of a spongy mass.

Example II A cell similar ot that employed in Example I was used in this example except that the cathode comprised a titanium wire inserted into the catholyte layer through a side tube. The neoprene stopper was replaced by a ground joint through which the carbon anode was inserted by means of a neoprene slip-joint.

The cell was heated to about 600 C. and electrolysis started. As in Example I, the upper layer quickly became water-white and only chlorine was evolved at the anode. The cell carried one amp. at an applied voltage of Y7 volts. After 45 minutes the current was turned off and the back E. M. F. measured. This amounted to 1.75 volts. The current was then measured as a function of voltage and from the plot, two inflections were noted at 1.75 volts and 2.5 volts. The cell voltage was then set at 2.4 volts and the electrolysis continued for about 2 /2 hours. Cell current at this voltage was 0.3 amp., giving a cathode current density of 4 amp. per sq. dm.

After the electrolysis was terminated the cell and contents were allowed to cool. The cell was broken open when cool and the contents examined. The upper salt layer again was pure white and a sharp boundary between the salt layers could be observed.

The frozen catholyte adhering to the cathode was examined. Needles of titanium metal 11 appeared to have grown radially outward from the wire for a distance of about 1 mm. Around this cylindrical mass of metal was another concentric cylinder of pale green salt of about 0.5 mm. therein, and the remainder of the salt looked like the original catholyte.

The content of the cathode section of the cell was leached with dilute inhibited acid and the metallic residue separated from the solution. The titanium metal was in the form of shiny needles about 1 mm. long. There was also some spongy material and a small amount of fine metal powder.

The cathode product metal is a solid due to the fact that the operating temperature of the electrolysis is far below the melting point of titanium metal. The products obtained in the above experiments were in the forms of dendritic crystals, spongy masses of crystals, or the metallic particles. At the present time experience with molten salt electrolyses indicates that at very low cathode current densities smooth metal plating may be achieved, whereas with higher current densities dendrites and the other crystal forms will appear.

Since halogen gas is evolved from the anode there is a tendency for the electrolysis cell to polarize when steady direct current, such as from a battery source, is used for electrolysis; therefore, I prefer to utilize an undulating direct current, such as is obtained by rectifying alternating current or by superimposing an alternating source upon a direct current.

Agitation of each electrolyte The may be resorted to if concentration polarization becomes troublesome.

The voltage of the cell is held above the deposition voltage of the ions of the metal to be deposited. Voltagedrops through the electrolyte and at the anode must also be supplied. The cell voltage more or less determines the current drawn. The cathode current density depends upon the current drawn and the size of the cathode area.

In adapting the invention to practice, resort can be had to batch or continuous operations, according to the dc gree of mechanization employed. Continuous flow of fresh electrolyte into and used electrolyte from the cell is contemplated in a continuous type of operation as it is also in a batch operation wherein the electrolyte titanium content is decreased and then the operation interrupted to replenish the titanium content of the catholyte. Another batch method would be to deplete the bath of: the titanium content, but this has several drawbacks. The melting point of the catholyte rises and the carrier salt may decompose as its relative concentration increases, possibly allowing metallic sodium to deposit.

A fused salt electrolyte usually comprises a stable carrier salt, having a higher decomposition voltage than the metal to be deposited, which serves as the solvent for the salt of the metal to be electrolytically deposited. The formation of a milky opaque bath may indicate the presence of oxygen in the form of titanium oxides which promote the production of fine cathode crystals. Oxides are harmful also because they are often trapped in the cathode crystal deposit, causing hard, brittle, non-ductile metal.

Various methods are useful in collecting and separating the cathode product. The electrolysis can be interrupted at intervals, if desired, the used cathode removed, a clean one inserted, and the electrolytically deposited metal stripped off the removed cathode. readying it for reinsertion on the next cycle. The electrolysis also may be continued until the crystals slough off, and fall to the bottom of the cell. This latter product can be recovered by raking, dipping, or discharging a slurry of the molten salt and metal from the cell and separating the metal values by settling, screening, or filtration. The cathode depositremoved from the cell can be treated with molten carrier salt to wash the crystals substantially free of titanium salt content. The metallic product then would be suitable for purification operations such as leaching with an aqueous inhibited acid. After the product is obtained as a concentrate of metal with minor amounts of salt, a further step of complete separation and recovery is required. This can be accomplished by known methods practiced in the titanium metal recovery art, such as leaching, vacuum distillation, or vaporization purification treatments designed toafford recovery of the metal product in pure form.

Electrolytic cells useful herein can be constructed in accordance with techniques common to the molten salt art. Refractory type molten salt-resistant brick con struction, corrosion-resistant, or other metal vessels, and frozen linings of salts can be employed in the process. It is necessary to prevent contact of the heated materials used as electrolytes and the hot metal product with high concentrations of Water vapor or oxygen and it is useful to purge the cell with inert, unreactive gases such as argon, helium, or like rare gases during a starting period or when the cell is opened for the addition of electrolyte or recovery of metal product.

Suitable anode electrodes, such as carbon, graphite, tungsten, etc., are inert towards the halogen at the operating temperature of the cell. The titanium can be plated on cathode electrodes of diiferent metals, but for the electrowinning of titanium I prefer to utilize a titanium cathode so that the system and product will not be contaminated.

While a magnesium and sodium chlorides anolyte and sodium chloride plus titanium subchloride catholyte sysassesses tern are particularly effective and preferred for use herein as immiscible, fused molten salts for electrowinning titanium, other forms of halide salts or mixtures, including chlorides, bromides and iodides of sodium and magnesium are also contemplated for use. Similarly, halides of the type just mentioned of alkali metals (sodium, potassium, lithium) generally and of magnesium and the alkaline earth metals (barium, calcium, strontium) can also be used. Preferably, the same salt-forming halogen element is present in each of the compounds forming the fused salt system. While the diand tri-subchlorides of titanium are preferred for use, titanium subiodides and bromides, such as TiBr Til etc., are also contemplated as useful. Anolytes which are more dense than the catholyte are also considered to be operably useful with proper consideration being given for the collection of the anode halogen product. For example, the anode section can be disposed in a suitable sidearm in association with the cell, or a suitable immersed launder can be provided for collecting the anode product gas.

By this invention an eflicient and economic electrolytic system to produce titanium metal is provided wherein a simple type cell can be utilized without recourse to an electrolyte separation diaphragm. The immiscibility barrier serves the function of a diaphragm without the attendant serious drawbacks present in diaphragm employments. The anode and cathode products of the electrolysis are held separate from each other; the electrolyte is not contaminated by materials present in a diaphragm; and the electrical path is not greatly lengthened.

I claim as my invention:

1. A process for preparing titanium metal which comprises electrolyzing with an inert anode and a metal cathode a separate fused salt anolyte layer which is substantially free of titanium :subhalides and consists essentially of an alkali metal halide and alkaline earth metal halide mixture, said anolyte being immiscible in, forming an interface between, and different in density from a separate layer of a fused catholyte employed in the process consisting essentially of a salt solution of a subhalide of titanium in a halide selected from the group consisting of an alkali and alkaline earth metal, maintaining said salt layers at temperatures ranging from 450 C. to about 750 C., said anode in separate contact with said anolyte and said cathode in separate contact with said catholyte, and separately recovering the anode and cathode electrolytic deposition products.

2. A process for preparing titanium metal which comprises electrolyzing with an inert anode and a metal cathode a separate fused salt anolyte layer which is substantially free of titanium subchlorides and consists essentially of an alkali met-a1 chloride and alkaline earth metal chloride mixture, said anolyte being immiscible in, forming an interface between, and different in density from a separate layer of a fused catholyte employed in the process consisting essentially of a salt solution of a subchloride of titanium in a chloride selected from the group consisting of an alkali and alkaline earth metal, maintaining said salt layers at temperatures ranging from 450 C. to about 750 C., said anode in separate contact with said anolyte and said cathode in separate contact with said catholyte, and separately recovering the anode and cathode electrolytic deposition products.

3. A process for preparing titanium metal which comprises electrolyzing with an inert anode and a titanium cathode a' separate fused salt layer anolyte substantially free of titanium subchlorides and consisting essentially of a mixture of an alkaline earth metal chloride in an alkali metal chloride, said anolyte being substantially immiscible in, forming an interface between, and lighter in density than the separate fused catholyte layer employed in the system, said catholyte consisting essentially of a fused salt solution of a titanium subchloride having a chlorine-to-titanium ratio of between about 2:1 and 3 :1 in an alkali metal chloride, maintaining said salt layers at temperatures ranging from 450 C. to about 750 C., and said anode in separate contact with said anolyte and said cathode in contact with said catholyte, and separately recovering the anode and cathode electrolysis products.

4. A process for preparing titanium metal which comprises electrolyzing with an inert anode and a titanium cathode a separate fused salt layer anolyte substantially free of titanium subchlorides, and consisting essentially of magnesium and sodium chlorides, said anolyte being substantially immiscible in, forming an interfacebetween, and lighter in density than the separate fused catholyte layer employed in the system, said catholyte consisting essentially of a fused salt solution of a titanium subchloride having a chlorine-to-titanium ratio of between 2:1 and 3:1 in sodium chloride, maintaining said salt layers at temperatures ranging from 450 C. to about 750 C. and said anode in separate contact with said anolyte and said cathode in contact with said catholyte, and separately recovering the anode and cathode electrolysis products.

5. A process for preparing titanium metal through electrolysis which comprises forming a molten salt solution consisting essentially of 48 to mol percent of sodium chloride and a titanium subchloride having a chlorine-to-titanium ratio of about 2:1 to 3: 1, introducing said solution to and forming a separate catholyte layer thereof in the cathode section of an electrolytic cell containing a titanium cathode for contact therewith, floating on the titanium-bearing molten salt catholyte layer a molten salt anolyte substantially free of titanium subchlorides and consisting essentially of a separate, immiscible layer of molten salt solution of magnesium chloride and sodium chloride of concentrations between about 40 to 70 mol percent sodium chloride, contacting said anolyte with an inert anode, passing an electrical current at electrolyzing potential from the cathode to the anode while maintaining the molten layers of electrolytes at temperatures ranging from 450 C. to 700 C., and separately recovering the anode and cathode deposit products.

6. A process for preparing titanium metal through electrolysis which comprises forming a molten salt solution consisting essentially of 48 to 90 mol percent of an alkali metal chloride and a titanium subchloride having a chlorine-to-titanium ratio of about 2:1 to 3:1, introducing said solution to and forming a separate catholyte layer thereof in the cathode section of an electrolytic cell containing a titanium cathode for contact therewith, floating on the titanium-bearing molten salt catholyte layer a molten salt anolyte substantially free of titanium subchlorides and consisting essentially of a separate, immiscible layer of molten salt solution of an alkaline earth metal chloride and an alkali metal chloride of concentrations between about 40 to 70 mol percent alkali metal chloride, contacting said anolyte with an inert anode, passing an electrical current at electrolyzing potential from the cathode to the anode while maintaining the molten layers of electrolytes at temperatures ranging from about 450 C. to 700 C., and separately recovering the anode and cathode deposit products.

References Cited in the file of this patent UNITED STATES PATENTS 1,452,813 Pauling Apr. 24, 1923 FOREIGN PATENTS 615,951 Germany July 16, 1935 OTHER REFERENCES Australian Journal of Applied Science, vol. 2, No. 3, September 1951, pages 358 thru 367, article by Cordner et al. 

1. A PROCESS FOR PREPARING TITANIUM METAL WHICH COMPRICES ELECTROLYZING WITH AN INERT ANODE AND A METAL CATHODE A SEPARATE FUSED SALT ANOLYTE LAYER WHICH IS SUBSTANTIALLY FREE OF TITANIUM SUBHALIDES AND CONSISTS ESSENTIALLY OF AN ALKALI METAL HALIDE AND ALKALINE EARTH METAL HALIDE MIXTURE, SAID ANOLYTE BEING IMMISCIBLE IN, FORMING AN INTERFACE BETWEEN, AND DIFFERENT IN DENSITY FROM A SEPARATE LAYER F A FUSED CATHOLYTE EMPLOYED IN THE PROCESS CONSISTING ESSENTIALLY OF A SALT SOLUTION F A SUBHALIDE OF TITANIUM IN A HALIDE SELECTED FROM THE GROUP CONSISTING OF AN ALKALI AND ALKALINE EARTH METAL, MAINTAINING SAID SALT LAYERS AT TEMPERATURES RANGING FROM 450*C. TO ABOUT 750*C., SAID ANODE IN SEPARATE CONTACT WITH SAID ANOLYTE AND SAID CATHODE IN SEPARATE CONTACT WITH SAID CATHOLYTE, AND SEPARATELY RECOVERING THE ANODE AND CATHODE ELECTROLYTIC DEPOSITION PRODUCTS. 